# What is it

Hand held, battery powered device for determining impedance of inductors (inductance), capacitors (capacitance) and resistors (resistance)

May also measure other things like inductor Q, capacitor leakage, series resistance (ESR of capacitors, DCR of inductors), etc.

# Theory

An LCR meter measures, among other things, the impedance, inductance, capacity and resistance of a Device Under Test. Usually the DUT is connected to an AC voltage source, then the voltage over and the current through the DUT are measured. The measured impedance consists of real and/or complex component. The phase angle (Φ) also is an important variable.

So we need a sine generator, voltage and current measurement, microcontroller, user interface and power supply.

# Characteristics

• Resistance range: from 1Ω to 1MΩ
• Capacity range: from 1pF to 1mF
• Inductance range: from 1μH to 1mH
• Measurement frequencies: 100Hz, 1kHz, 10kHz
• Measurement voltage: 500mVrms
• Offset voltage for measuring electrolytic capacitors
• DC voltage for capacitor leakage current measurement

# Sine generator

There are some 'instant' sine generators that are not suited for the openbench function generator, but might be suitable for this application. All that is needed is a fixed output level and a couple of frequency presets (100Hz, 1kHz, 10kHz).

# Voltage and current measurement

Voltage is measured over the DUT, current is measured as voltage over a shunt resistor. The values are amplified, digitized and sent to the microcontroller. Because of the wide variations in voltage and current, a low noise amplifier is needed, preferable with programmable gain. An instrumentation amplifier is perfect for this. The analog values are then digitized using an A/D converter.

What would be the voltage and current in case of the biggest or smallest capacitor or inductor or resistor? What resolution and sample rate do we need for the ADC?

Analog Devices has some nice 'instant' instrumentation amplifiers, even including programmable gain.

# User interface

The display must be able to show inductor/capacitor/resistor, series/parallel configuration, L, C, R, Φ, Q and D readout. The user must be able to select measurement frequency, voltage bias and calibrate the meter.

# Power supply

What voltages do we need? Probably digital 5V and analog +/- 5V.